Well, it depends what definition you use. Christians used to, and sometimes still do, call all other religions 'heathen', 'pagan' or even 'satanic' (normally a term reserved for actual pagan religions). Drastically simplified, paganism is a family of faiths centered (at least nowadays) upon the worship and/or deification of nature, personified or otherwise, which puts it apart from new-age or occultic faiths centered mainly on personal spirituality or magic. At least thats how I use it. For example, wicca is pagan, Thelema and LaVey Satanism aren't.
Its more complex than that of course. I mean, for example, there are three types of paganism (paleopaganism, mesopaganism and neopaganism) and there's heathenry, and all that palava. And crossovers. It's a darned mess.